Digital relay advantages and functional blocks

A digital (numerical) relay samples the analog CT/PT signals, converts them to digital numbers and uses a microprocessor running protection algorithms to decide whether to trip.

Advantages

1. Multi-function: one relay gives overcurrent, earth fault, distance, differential, under/over-voltage, frequency, etc.
2. Self-monitoring of hardware and software; failure gives an alarm.
3. Flexible: characteristics and settings changed by software; several setting groups.
4. Very low CT/PT burden and high accuracy; no drift with temperature.
5. Fault recording: event log, disturbance (waveform) record and fault location.
6. Communication: SCADA/IEC 61850 links, remote setting and reading.
7. Compact, lower cost per function, less panel space and wiring.
8. Adaptive protection possible (settings change with system conditions).

Basic functional blocks

CT/PT
  │
[Isolation & signal conditioning
 (aux CT/PT, surge filter)]
  │
[Anti-aliasing low-pass filter]
  │
[Sample & hold]──[Analog MUX]
  │
[A/D converter]
  │
[Microprocessor / DSP] ◄─► [RAM, ROM/EPROM,
  │  (algorithm)              EEPROM settings]
  ├─► [Digital output → trip relay → CB]
  ├─► [Display, keypad, LEDs]
  └─► [Communication port]
  ▲
[Digital inputs: CB status, etc.]
[Power supply (DC-DC from battery)]

1. Signal conditioning: auxiliary transformers scale CT/PT outputs to low voltages (±10 V) and give isolation; surge filters protect electronics.
2. Anti-aliasing filter: low-pass filter removes frequencies above half the sampling rate (Nyquist).
3. Sample-and-hold and multiplexer: freeze each channel's value at the sampling instant and pass channels one by one.
4. A/D converter: converts each sample to a binary number (12–16 bit).
5. Microprocessor/DSP: runs algorithms (Fourier, Walsh, least-squares) to find phasors, impedance, etc., compares with settings and decides.
6. Memory: ROM for program, RAM for samples, EEPROM for settings and records.
7. Digital inputs/outputs: read CB status; output trip and alarm contacts.
8. HMI and communication: display, keypad, and ports for remote access.
9. Power supply: isolated DC-DC converter from the station battery.